The invention relates generally to visualizing anatomical images. More specifically, the invention relates to a method and apparatus for determining the position of a probe relative to various anatomical features and displaying the internal anatomical structures corresponding to the position of the probe.

BACKGROUND OF THE INVENTION In recent years it has become commonplace for a surgeon to utilize slice images of a patient's internal organs. The images are used to plan the course of a medical procedure, be it diagnostic, therapeutic, or surgical, and for orientation during the procedure. The slice images are typically generated by Computerized Tomography (CT) or by Magnetic Resonance Imaging (MRI). Images may also be captured using Angiography, Single- Photon Emission Computed Tomography, and Positron Emission Tomography methods. The images typically presented to a user consist of a series of static images on film. These images are very detailed and can resolve anatomical structures less than one millimetre in size. However, their format differs greatly from the actual anatomical features seen during the surgical procedure. The images are presented in two-dimensional form rather than in the three- dimensional form of the anatomical features. In addition, the perspective of the slice image rarely corresponds to the surgeon's viewing angle during the procedure. Consequently, during a procedure, the slice images provide a primitive visualization aid to the patient's anatomy.

To obtain proper orientation within a patient's body, surgeons can make an incision which is larger than the minimum required for the planned procedure. "While providing an enlarged window to the patient's anatomy, these larger incisions may result in longer hospital stays

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and increased risk for the patient. On the other hand, if only a small incision is made, the field of view available to the surgeon is greatly limited. As a result, the surgeon may become disoriented forcing him to correct and recommence the procedure, or to continue at a high risk to the patient.

While imaging equipment can be used to provide on-the-spot visualization of a patient, it is impractical to use the equipment in the operating room during the procedure. First, the costs of purchasing, operating and maintaining the imaging equipment are prohibitive. Secondly, surgeons have limited access to a patient who is placed in a scanning device. Furthermore, Magnetic Resonance Imaging and Computerized Tomography have side effects which may harm the patient and inhibit the procedures. Magnetic Resonance Imaging produces a very high fixed magnetic field which precludes the use of many instruments. Computerized Tomography, on the other hand, utilizes X-ray radiation which is known to damage human tissue and cause cancer. It is, therefore, not desirable to expose a patient to a computerized tomography scan for a prolonged period. A known approach to localizing anatomy during surgery is currently being used for brain lesions. The method is known as stereotactic surgery. It involves rigidly attaching the reference frame to the patient*s head during the scanning. Using the marks left in the scanned images by the frame, the location of the lesion is computed. During the surgical procedure, a reference frame is again attached to the same location on the patient's head. The frame is used to direct drilling and cutting operations, which are done either manually or automatically.

Stereotactic surgery has a number of drawbacks. Firstly, it is only suitable for localized brain lesions which have a direct approach path. Secondly, stereotactic surgery requires the use of a cumbersome and uncomfortable

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reference frame. Furthermore, since the decision to undertake stereotactic surgery is usually done after a first scanning procedure, the patient must undergo a second scan with the frame attached. This results in a prolonged and expensive procedure. Moreover, if the scan utilizes computerized tomography imaging, then the patient is exposed to another.dose of radiation.

Known in the art are systems and methods designed to allow the use of previously acquired Computer Tomography or Magnetic Resonance Imaging scans as an aide in conventional open neurosurgery. In general, these methods use systems comprising:

(a)a multi-jointed probe or sensor arm;

(b)a computer processing unit which calculates the position of the probe arm relative to certain reference points on the patent; and

(c)a means of displaying the superpositioning of the location of the probe arm as calculated above on the previously acquired scan images. The display capabilities of such systems are limited in that they can display only the slice images as generated by the computerized tomography or magnetic resonance imaging scan. An example of such a system is disclosed in an international Application filed by Georg Schlδndovff and published under No. WO88/09151. This application is mainly concerned with an arm structure for locating the position of a probe.

SUMMARY OF THE INVENTION

In a first aspect the invention provides a method for visualizing internal regions of an anatomical body in relation to a probe, employing a data-base body of previously acquired images of the anatomical body, the method comprising the steps of:

(a)obtaining a spatial position for the probe relative to the anatomical body;

(b)determining a data-base location relative to

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the data-base body corresponding to the spatial position of the probe relative to the anatomical body;

(c)mapping the spatial position of the probe relative to the anatomical body to the corresponding data- base location of the probe relative to the data-base body; and

(d)displaying a region of the data-base body adjacent the data-base location of the probe.

In a second aspect the invention provides a system for visualizing internal regions of an anatomical body by utilizing a data-base body of previously acquired images of the anatomical body, the system comprising:

(a)a probe;

(b)a data-base storage unit containing the previously acquired images of the anatomical body;

(c)a spatial determinator for determining the spatial position of the probe relative to the anatomical body;

(d)a computer using the previously acquired images to generate a representation of a region of the anatomical body adjacent to the spatial position of the probe; and

(e)a display unit for displaying the representation of the anatomical body.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying drawings which show alternate embodiments of the present invention, and in which:

Figure lis a first embodiment of a probe- correlated imaging system;

Figure 2is a portion of a second embodiment of a probe-correlated imaging system; Figure 3is a portion of a third embodiment of a probe-correlated imaging system;

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Figure 4is a first display format employed in the system of fig. 1;

Figure 5is a second display format employed in the system of fig. 1; and Figure 6is a third display format employed in the system of fig. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to fig. 1 a probe-correlated system (1) has a probe (10), a computer (11), a data storage unit (12), and a display (13). These components are, individually, well known and common. The system (1) is employed to view the anatomical structure of a patient (9) adjacent to the position of the probe (10).

The computer (11) has ready access to the unit (12) which contains a data-base body (17) representing the anatomical structure of the patient (9). The data-base body (17) includes previously acquired digital images (15) of the patient (9). These images (15) can be acquired through various medical-imaging techniques, such as Computerized Tomography, Single-Photon Emission Computed Tomography, Positron Emission Tomography, Magnetic Resonance Imaging, Ultrasound, or Angiography.

In addition to the digital images (15) captured by medical-imaging techniques, the data-base body (17) can contain pre-processed digital images (16). For example, the digital images (15) together with their relative spatial relationship can be pre-processed to represent the various organ surfaces of the patient (9). There are known systems, not shown, which can read digital images (15) and generate pre-processed digital images (16) according to their relative spatial relationship within the anatomical structure of the patient (9). The known system places the pre-processed images (16) in the data¬ base body (17). The probe (10), or any other object which may function as a probe, is used by an operator, not shown, to point to a particular location on the anatomical

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body of the patient (9). The operator can move the probe (10) around or within the anatomical body of the patient

(9)-

Spatial coordinates, representing the spatial position and possibly the spatial orientation, of the probe relative to a fixed reference point, shown generally at the arrow (20), are conveyed to the computer (11). The reference point (20) may either be on the patient (9) as shown, or on some stable platform nearby, not shown. There are a number of alternate methods which can be used to obtain the spatial coordinates of the probe (10) relative to its reference point (20) . The apparatuses described in association with such method will be collectively referred to as spatial determinators. Referring to fig. 1, an electro-magnetic emitter

(20a) is positioned at the reference point (20) and a sensor (20b) is located on the probe (10). By comparing the timing and phase of transmitted signals from the emitter (20a) to received signals picked up by the sensor (20b), the position and orientation of the probe (10) relative to the reference point (20) can be determined. A probe (10) using this known locating method is commercially available. Given the spatial relationship between the reference point (20) and the patient (9), the computer (11) can determine the position of the probe (10) relative to the patient (9) .

Alternately, referring to fig. 2, the probe (10) is attached to a multi-joint light-weight arm (25) with a first section (26) and a second section (27) connected together at joint (22). The first section (26) of the multi-joint arm (25) is connected to a base (28) at joint (21). The base (28) is attached to the patient (9) using adhesive elastic tape (23). The probe (10) is attached to the second section (27) at joint (24). The joints (21), (22), (24), in combination, provide for a range of motion equal to or greater than that required for a given procedure. Angular sensors, not

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shown, are located at the joints (21), (22), (24).

The angular sensors are connected by wire (28a) to one another and to an electronic unit (29). The sensors detect any change in the position or orientation of the multi- oint arm (25), and convey this information to the electronic unit (29).

The unit (29) uses geometric calculations to determine the spatial position and spatial orientation of the probe (10) relative to the base (28) which is used as the reference point. The spatial position and spatial orientation of the probe (10) are sent to the computer (11) of fig. 1 through an electronic communication link (27). A suitable communication link (27) would be an RS- 232 serial communication interface. Since the base (28) is fixed to the body of the patient (9), the computer can use the spatial information to determine the position of the probe (10) relative to the patient (9).

Alternately, referring to fig. 3, a dual-arm arrangement, shown generally at (31), may be employed. The arrangement (31) is particularly effective where the multi-joint arm (30) of fig. 2 cannot be fixed to the patient (9) .

A stand (35) is used to anchor two multi-joint arms (36, 37) similar to the multi-joint arm (30) of fig. 2. A probe (10) is attached to the other end of arm (37). Arm (36) is attached at its other end to a reference point (40) on the patient (9). Sensors are mounted at joints (41, 42, 43) of arm (37), and at joints (44, 45, 46) of arm (36). The sensors, in turn, are connected to an electronic unit (39). The electronic unit (39) decodes the position and orientation of the probe (10). Through the relative spatial positions and orientations of the probe (10) to the joint (41), the joint (41) to the joint (44) and the joint (44) to the reference point (40), the spatial position and orientation of the probe (10) relative to the patient (9) is obtained. The spatial position and orientation of the probe (10) is transmitted

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to the computer (11) of fig. 1 via the communication link (47).

The reference arm (36) shown in fig. 3 can be omitted if the patient (9) is fixed to an operating table (48). The patient can be fixed to the table (48) using straps (49). If the patient (9) is fixed, then the reference point (40) can be fixed arbitrarily in space. The relative position of the reference point (40) to the joint (41) may be determined once and the relative position of the probe (10) to the reference point (40) determined therefrom. However, if the patient (9) is moved during the procedure, a new reference point (40) or a new spatial relationship must be established.

To display the data-base image (15) or pre- processed image (16) which correctly corresponds to the region of the anatomical body of the patient (9) adjacent the probe (10), the system (1) must be able to map positions of the anatomical body of the patient (9) to locations in data-base body (17) during the procedure. In this sense mapping is a procedure for determining the current spatial position of the probe (10) and the corresponding adjacent data-base body (17) location. This correspondence may be initially determined through a procedure which maps the patient (9) to the data-base body (17). This procedure is known as "registration" since its purpose is to register the correspondence between the anatomical body of the patient (9) and the data-base body (17) with the computer (11).

A number of registration methods are known. For example, one method involves the marking of reference points on the patent (9). However, this can be inconvenient and there is a risk that the marked positions on the patient (9) may be erased between the time the scan images (15) were generated and the time the surgical procedure is performed. Another method involves placing small markers, usually made of cad or ceramic material, on readily identifiable features of the patent, such as the

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ears or the corners of the eyes.

The preferred registration method involves using the probe (10) to register with the computer (11) the spatial position of easily identifiable features of the patient, such as the space between the teeth, the nose or the corners of the eyes. In this method, the previously acquired scan images (15) or the pre-processed images (16) are displayed on the display (13) in such a manner as to allow the user of the system (1) to identify specific points of the chosen features of the patient (9). A three dimensional surface format, shown in figure 6, is the simplest such format for an unskilled viewer to comprehend. Such a three-dimensional surface format can be derived from the pre-processed images (16) in a known manner, and suitable points such as the corners of the eyes (70), space between the teeth (72) are shown in figure 6.

The method is as follows. The probe (10) is placed next to the feature point on the patient (9) . The spatial position of the probe (10) is then determined. A movable marker, e.g. a cursor, on the display (13) is then adjusted so it coincides with a selected feature, e.g. corner of the eyes (70) as seen. It is then relatively simple for the computer (11) to perform necessary three dimensional transformation, so that the spatial position of the probe (10) and the corresponding data-base body location are registered with the computer (11). Using a set of at least three, and preferably about six, feature points on the patient, a proper and unique transformation function, can be calculated which maps the spatial position of the probe (10) to the corresponding data-base body location and orientation. The accuracy of this transformation function is improved by the use of a larger number of points and a statistical error minimizing techniques, such as the least mean square error method.

Once the anatomical body of the patient (9) has been registered with the computer (11), the operator can

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move the probe (10) in and around the patient (9), and at the same time view the hidden anatomical features of the patient (9) as they appear in the data-base body (17). The anatomical features of the patient (9) in the data- base body (17) are presented on the display unit (13) in relationship to the spatial position and possibly orientation of the probe (10).

It is not strictly necessary to use the orientation of the probe (10) to carry out many of the features of the invention. The probe (10) may be represented on the display (13) as a point rather than a full probe (10). The region adjacent the point probe (10) is then displayed. The orientation of the regions displayed is known from the computer (11) and not determined by the orientation of the probe (10).

A possible presentation format for the data-base images (15) of the patient (9) is shown in fig. 4. Two- dimensional representations or slice images are generated by the computer (11) from the data-base images (15). The position of the probe (10) relative to the anatomical body (9) is marked on a slice image (50) by the computer (11). The slice image (50) together with the probe (52) are displayed on the unit (13).

The screen of the display unit (13) is divided into 4 separate windows. Three of the windows contain slice images corresponding to three cardinal anatomical planes: sagittal (50); axial (54); and coronal (56). The three slice images (50, 54, 56) intersect at the location of the probe (52). Thus, the operator can observe the anatomical feature of the patient (9) relative to the position of the probe (10) in the six main directions: anterior, posterior, superior, inferior, right and left. The fourth window depicted on the display unit (13) can show a slice (57) through the anatomical features in mid- sagittal orientation along the axis of the probe (10). The position and orientation of the probe (10) can be marked on the slice (57), thereby allowing the operator to

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direct viewing of what lies ahead of the probe (10).

Another presentation format for the data-base images (15) and pre-processed images (16) is shown in fig.

5. A three-dimensional model (58) of the patient (9) is generated by the computer (11) from the images (15, 16).

The computer (11) also generates a three-dimensional model (60) of the probe (10). The relative locations of the models (60), (58) correspond to the spatial position and orientation of the probe (10) relative to the patient (9). The three-dimensional model (58) of the patient (9) generated from the stored images (15, 16) is presented together with the model (60) on the display unit (13).

Other display methods than the display of slices or using pre-processing may be used in conjunction with the principles described herein. For example, the computer (11) can generate displays directly from the images (15) using a ray-cast method. In one ray-cast method the computer (11) creates the display using the results of simulated X-rays passing through the images (15). The simulated X-rays will be affected differently by different elements in the images (15) according to their relative absorption of the X-rays. The results may be displayed along with the probe (10) in a manner similar to those described for slices or 3d-images. This produces a simulated X-ray display. In another ray-cast method a display is created using the results of simulated light rays passing through the images (15). The elements in the images (15) which do not pass the simulated light rays correspond to surface features and may be used to generate a display similar to the three-dimensional model (58).

There are other ray casting methods which are well known in the art.

The computer (11) can be used to further process the slice image (50) and three-dimensional images (58) generated from the data-base body (17). For example, a wedge-shaped portion (62) has been cut from the three- dimensional image (58). The cut-out portion (62) exposes

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various structures adjacent to the probe (10), which would not otherwise be observable. In addition, the cut-out portion (62) gives the operator an unobstructed view of the position of the probe (10) even if it is within the patient (9). The slice images (50) and three-dimensional images (58), (60) can also be processed by the computer (11) using other known image processing techniques. For example, the model (60) of the probe (10) can be made translucent, or the slice image (50) can be combined with other slice views.

While the present invention has been described with reference to certain preferred embodiments various modifications will be apparent to those skilled in the art and any such modifications are intended to be within the scope of the invention as set forth in the appended claims.

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